The rise of Napa began with an upset. Warren Winiarski would know – his wine, a cabernet sauvignon, was a firm underdog at a legendary 1976 blind tasting in Paris, which pitted the best of France against the little-known California region.

His winery, Stag’s Leap, shocked the wine world by taking top honors. “It broke the glass ceiling that France had imposed on everyone,” he recalls. “People’s aspirations were liberated.”

Today Winiarski, 89, is speaking not of liberation, but of limits. A growing coalition of industry veterans and longtime residents fear that Napa has become a victim of its own success, pointing to the ecological transformation of the valley floor from dense oak woodland to a sea of vine-wrapped trellises. And they are posing a thorny question: has a unique agricultural region reached a tipping point at which agriculture itself becomes the threat?

“We’re not thinking ahead,” says Winiarski. “What’s at stake is a national treasure.”

Against this backdrop, a local environmental initiative has sparked fierce debate. The effort, known as Measure C, would cap the amount of oak woodland that could be cleared for future vineyards – in effect limiting the growth of some of the world’s most famous wine brands.

Napa Valley is small, just 30 miles long and five miles wide. Nearly 500 wineries now call this slim stretch home; it welcomes 3.5 million visitors a year. Global recognition has attracted big beverage companies. Tourists have clogged the narrow two-lane roads. Wealthy “lifestyle vintners” have scooped up second homes and attempted to build private helicopter pads.

“With great success came great money and outsiders,” explains James Conaway, a journalist and author who has been covering Napa since the 1980s. He describes the valley of 30 years ago as egalitarian and idealistic, a mixed agricultural community that raised wine alongside livestock and fruit trees. “Now it’s monoculture with a vengeance. Hundreds of miles of steel trellising holding up the vines from one end of the valley to the next. It has an industrial sheen.”

Through the windows at Winiarski’s hilltop home, the transformation can be surveyed with ease. Vineyards stretch in all directions, rows of green as orderly as soldiers. The Silverado Trail, a famous wine tasting route, cuts a path to the west. To the east lie hills covered in oaks – trees that, Winariski points out, once carpeted the valley floor.

Napa’s oaks have become a flashpoint in the story of wine’s takeover. Ninety-five per cent of those on the valley floor have been felled, the vasty majority replaced by grapes. Now developers are eyeing the surrounding hillsides.

Napa county has California’s densest concentration of oak woodland, thanks to the rich foliage that still carpets the hills. While much of it is privately owned and not public land in the classic sense, the woods are regarded as a public resource – a place of recreation and biodiversity, a vital part of the valley’s watershed and a fierce point of pride. But more than a third sits on potentially agriculturally productive soil – a 2010 management plan estimated that by 2030, up to 3,065 acres of mixed woodland would be lost due to vineyard development.

“Forests are the best negative emissions technology we have,” says Jim Wilson, a former brewing quality manager at Anheuser-Busch and a leader in the band of grassroots activists behind Measure C. “Should Napa’s wine industry get a pass?”

The measure is the culmination of years-long battle – one that’s involved knocking on doors, gathering thousands of signatures, and fighting an opposition which, according to a private newsletter seen by the Guardian, plans to spend nearly $1m to defeat it. Wilson’s side, by contrast, has raised just over $160,000. The effort has consumed his life, but then, Napa is his life.

“I was born here in 1955,” Wilson says. “I raised a family on my wife’s ranch.” Their home – a patchwork of steep hills, creeks and woodlands on the county’s east side – is wild and uncultivated. During a walk beneath the oaks on a recent morning, his love for the forest is palpable.

“When you take forest out, you negatively impact carbon sequestration,” he explains. The trees play a crucial role in capturing rain and replenishing groundwater, he said, while their root systems prevent soil erosion.

Voters will decide on the proposal by 5 June, but the campaign has sharply divided the wine community. Veteran vintners like Winiarski have gone against the industry trade groups, who are united in their opposition. The Napa Valley Vintners, a key trade body, initially supported the measure but later pulled a surprise U-turn. Wilson and his co-organizers say pressure from powerful industry figures turned the tide. The Vintners declined to comment on the reversal, but an official statement said the majority of its members “conveyed opposition”.

“This decision does not change our longstanding commitment to promoting, protecting and enhancing the Napa Valley and to upholding its goals of advocating for the local wine industry while preserving this special place for future generations,” it added.

Ryan Klobas, the policy director for the Napa Valley Farm Bureau, warns the measure is “full of unintended consequences” and calls its proposed regulation too complicated for voters to grasp. “You’re asking everyone to become a technical expert overnight. This is an issue better left to the board of supervisors.”

But longtime residents such as Ginny Simms – an environmental advocate who served on the county’s board of supervisors in the 1970s – believe corporate greed is at the root. Napa today is a multibillion-dollar industry, where global beverage companies such as Treasury Wine Estates, Gallo and Constellation – which own brands such as Corona and Svedka vodka – have acquired smaller wineries for their portfolios.

“The real issue here is power,” says Simms, 90. “Opponents want to run the county in a way that is favorable to the expansion of all wineries and wine events. Which leaves the people of Napa voiceless.”

Despite the talk of pushing back against power, some worry more regulation would actually favor the wealthy by boosting the price of the little free land that remains. Don Clark is a mid-sized grape grower and vineyard manager from Texas, who came to Napa in 1994 and was lucky to buy affordable land.

“We may have been the last generation who could come here as a young farmer,” he says. “The barriers to entry are almost impossible now.” Startup costs have soared – Clark’s last vineyard development client spent half a million dollars on various legal, consultation and county fees, as well as archeological and environmental studies, he says.

Clark and others point to a landmark 1968 ordinance known as the Agriculture Preserve – the first of its kind in the US – which deemed agriculture the “highest and best” use of Napa’s land. Measure C therefore undercuts a fundamental right to farm, these opponents argue.

But proponents say the measure is born of love, not reproach, for the wine world, and is simply about responsible farming. “Something’s very wrong with the way we are thinking about our resources,” Winiarski says. “They are finite. And yet we go on with development as though we could do that indefinitely.”